Tennis now is different.
The slowness of conditions + poly strings (that allow to impart more spin on the ball and so the shots are safer than before, baseline tennis easier than ever before) makes tennis now more of an endurance sport, with long to very long baseline exchanges 80% of the points.
Like it happens in marathon races, triathletes,....we will see more and more 30-something tennis players doing well in tennis from now on.
You may lose explosiveness of movement, reflexes, reaction time, recovery time... with age but not endurance. In fact you can endure more (if well trained) at 30-something than at 20-something.